The relation of dimer size, double-stranded, circular DNA of the replicative form of the bacterial viruses phi X174 and G4 to genetic recombination is being investigated. A dimeric intermediate, the figure 8, has been identified by electron microscopy that consists of one dimer-length and two monomer-length circular strands in the form of a covalently-closed duplex. The figure 8 is a configuration in which branch migration can occur. This process has been shown to be obligatory for recombination in the phi X-G4 system. The properties of the figure 8 are being studied by biochemical, genetic and physical methods including electron microscopy and analytical ultracentrifugation. These studies will make use of isolated figure 8s and their infectivity in spheroplast tests of recombination. They will attempt to define the in vivo role of the figure 8 in recombination and its conversion or other relation to circular and catanated dimers. A configuration of DNA, the X-form, derived from the figure 8 of phage G4 by treatment with the Eco Rl restriction enzyme has made available a DNA with which the kinetics of branch migration are being measured and factors evaluated that determine the rate and significance of the process in vivo. The kinetics of renaturation of denatured, covalently-closed, double-stranded, circular DNA is being investigated. Initial work on phi X174 replicative form has been extended to G4 replicative form, PM2 DNA and a series of plasmids. Studies on the mechanism of the reaction and on the influence of the conditions of denaturation on the rate of renaturation are being emphasized.